Seasons
are determined by the way that your
planet orbits its sun. Find
out how.

A planet spins about an axis like a bead on
a toothpick.

The axis is an imaginary line that goes through both of the
planet's poles. The planet rotates around this line.

The planet also revolves around the sun in a path called its
orbit. The orbit is elliptical (see Kepler's
First Law). Imagine the orbit as a flat piece of paper
with the planet moving along its outside. This is called the
plane of the planet's orbit.

The axis may be tilted in relationship to the orbital plane.
The axis may be straight up and down or tipped over to one side.
The position of the axis influences the climate and weather of
the planet. Let's take a few minutes to think about this.

Like the earth, this planet has an axis that is tilted.
Note the effect that this has on the planet.

In the first position, it is summer at the planet's north
pole. The north pole is getting sunlight with no darkness at
night because it is tilted towards the sun. the south pole is
having winter, as it is in darkness all the time.

In the second position, the planet is having its autumn equinox
-- the day when the daylight and darkness last for the same length
of time. If this is the earth, every part of the earth is getting
twelve hours of light and twelve hours of darkness.

In the third position, the north pole is now tipped away
from the sun. The north pole is now in darkness, while the lighted
south pole is experiencing the very long summer days.

In the fourth position, the planet is experiencing the spring
equinox. Again, the length of the day and the length of the night
are equal everywhere on the planet.

However, the temperatures
are not even all over the
planet, and, at the poles,
even the long days of summer
are not so warm as days in
the tropics. Why is this?

Look at this diagram
carefully.

See how the dark yellow rays of the sun strike the earth.
The earth is curved here, so that the rays spread out a long
way over the slanted surface.

In the central portion, the rays of the sun strike the earth
more directly. This means that each little part of the earth
here gets more direct light. Therefore, it also gets more energy
and more heat.

Notice how, at the lighted pole, the sunlight is also spread
out. Even though the days are long and the nights very short,
the weather does not get really hot here because the sunbeams
are spread out over a greater area of ground that in the hot
tropical regions. The sunlight here is simply not as intense
as it is closer to the center of the planet.

Now let's look at some other possibilities.

With this planet, the axis is
straight up and down. As this planet travels around its sun,
every day is the same length. The planet is rotating at a constant
speed, and all parts of the planet receive light.

However, the temperature is not even all over this world.
Study the diagram above again.

This planet will receive direct light at its equator, and the
weather will be hot from receiving all that energy.

However, up toward the poles the curvature of the planet will
again cause the rays of the sun to be more spread out. Parts
of this planet near the poles will be cooler than the temperatures
at the equator.

An
exception to the No
Seasons rule will occur
if the planet has an
elliptical orbit. Then
the planet would be nearer
the sun on part of its
journey and farther from it
at other times. On
this planet the temperatures
would be warm when the
planet is near the sun and
colder when the planet is
farther away. The
whole planet would be warmer
or cooler at the same
time. What does this
mean?

Plants
would adapt to the cooler
time as they do on earth, by
shutting down during
inhospitable times (due to
extreme heat, drought, or
cold) and by making
seeds. Hibernation
would work for animals, but
migration might or might
not. This would depend
on the temperature range,

This planet looks as if it is lying down on its orbit, just
rolling along. What will the weather be like here?

(1) At one end of the orbit, half of the planet will be in
sunlight all the time. The pole will be right in the middle of
this sunlit part, and will be a hot place! Temperatures will
decrease in concentric circles as measurements are taken farther
and farther from the pole, and as the light covers more curved
parts of the planet. The other side of the planet will be freezing
in the dark.

(2)Midway along the orbit, where earth experiences equinoxes,
the planet's axis will be parallel to the orbital path. The planet
will be turning like a roast on a spit. Every part of the planet
will get equal hours of light and darkness every day. The hottest
part will be along the equator, midway between the poles, because
that part of the planet will receive the most direct solar rays.

(3) This position, opposite (1) will be exactly like (1) except
that the other pole will now be hot. The pole that was hot before
is now in freezing darkness.

(4) This position will be exactly like (2). Days and nights
will be equal, and every part of the planet will receive light
every day.

If you want to do a planet with a day like this, use a ball
and maybe a flashlight to make a model. Talk with Dr Viau about
this. It is very different from what we know.